1. Field of the Invention
This invention relates to a transistor, more particularly to an organic thin film transistor.
2. Description of the Related Art
An organic thin film transistor (OTFT) is flexible, easy to manufacture, and cost effective, and thus, many researches have focused on this field in recent years.
Referring to FIG. 1, an OTFT 1 generally includes a gate insulating film 11, a source electrode 12, a drain electrode 13, a gate electrode 14, and an organic active layer 16. The source and drain electrodes 12, 13 are respectively disposed on an upper surface of the gate insulating film 11. The organic active layer 16, which includes an organic semiconductor material, is disposed on a channel portion 15 cooperatively defined by the source and drain electrodes 12, 13 and the gate insulating film 11 and is in contact with the source and drain electrodes 12, 13.
For disposing the organic semiconductor material of the organic active layer 16 onto the OTFT 1 with a large area, an ink-jet printing method is commercially preferable in view of cost effectiveness and operation convenience. The ink-jet printing method is conducted by dissolving the organic semiconductor material in a solvent to form a solution, and printing the solution onto an element (i.e., onto the gate insulating film 11 in the channel portion 15) using an ink-jet printer, followed by drying. For the ink-jet printing method, the organic semiconductor material used in the formation of the organic active layer should have a relatively high solubility in the solvent and high stability in the atmosphere.
Examples of the conventional organic semiconductor material used in the ink-jet printing method include pentacene published in M.R.S Bulletin., 2003, vol. 771, p 169, rubrene published in Nature, 2006, vol. 444, p 913, polythiophene (P3HT) published in Chemistry of Materials, 1998, vol. 10, p 457, trimethylsilyl-substituted quarterthiophene (TMS-4T) published in Advanced materials, 2008, vol. 20, p 4044, and so on. The conventional organic semiconductor materials of P3HT and TMS-4T are expectedly suitable for used in the ink-jet printing method.
Since the above-mentioned organic semiconductor materials are all hydrophobic compounds, a hydrophobic layer 112 is preferably provided between the gate insulating film 11 and the organic active layer 16 in the channel portion 15, and each of the source and drain electrodes 12, 13 is designed to be made of a hydrophilic material. Accordingly, the hydrophobic organic semiconductor material can be stably positioned in the channel portion 15, and after the same is dried, the crystal thereof can exhibit a superior crystal quality. The hydrophobic layer 112, for example, can be made of hexamethyl disilazane (HMDS) or octadecyltrichlorosilane (OTS).
However, there are several problems with respect to the above-mentioned hydrophobic organic semiconductor materials. First of all, a hydrophobic organic solvent has to be used in dissolving the aforesaid hydrophobic organic semiconductor materials. Certain hydrophobic organic solvents are likely to damage the environment and can cause health concerns to users, and thus, the equipment and space for operation of the hydrophobic organic solvents should be strictly controlled. Moreover, certain hydrophobic organic solvents are likely to damage the organic insulating films, such as PVP (poly-4-vinylphenol) in OTFTs and thus, the deposition of the organic active layer 16 using the ink-jet printing method is impeded.
In addition, the above-mentioned organic semiconductor materials are likely to be attacked by moisture or oxygen, and thus, cannot be stably stored in the atmosphere. For example, P3HT can be stored in the atmosphere for only one week, and pentacene is extremely unstable and cannot be used in the process for forming transistors under the atmosphere.